The present invention relates to an image processing method, an image processing apparatus and an image forming apparatus that perform a dither processing using a dither matrix based on the result of determination of the segmentation class to which the pixel to be processed belongs, and to a recording medium where a computer program is recorded for causing a computer to perform the dither processing using the dither matrix based on the result of determination of the segmentation class to which the pixel to be processed belongs.
In recent years, since the digitization of OA apparatuses has rapidly proceeded and the demand for color image output has increased, output apparatuses such as electrophotographic digital color copiers and inkjet and thermal transfer printers have become widespread. For example, image information inputted from input apparatuses such as digital cameras and image scanners and image information created on computers are outputted by using these output apparatuses. In these output apparatuses, a pseudo tone reproduction processing is frequently performed on the inputted image information.
A method of the pseudo tone reproduction processing is the multi-level dither method. For example, on the pixels of a dither matrix as shown in FIG. 1 (here, the numerals in the cells represent the order in which higher output values are assigned), quantization is performed based on the comparison of the pixel values and a plurality of corresponding threshold values, and one of a plurality of output values is outputted or an output value converted based on a conversion table is outputted.
A technology has been proposed such that by using a dither matrix of a concentrated type having a number, k (k≧2), of cores, forming a screen angle and making the frequency of occurrence of dots whose densities are not the highest level not more than k within the dither matrix, an improvement in reproducibility and the stabilization of image quality such as smooth tone representation are attained and by making the density level difference between dots not more than 1 by uniformly increasing the density within the dither matrix, resolution is maintained within a permissible range and the halftone dots are well-balancedly expressed to thereby prevent the screen angle from disappearing (see, for example, Japanese Patent Application Laid-Open No. 61-189774 (1986)).
For the pixels determined to be the edge area by the segmentation processing, in order to clearly reproduce characters and thin lines, unlike the multi-level dither processing for the area other than the edge area, for example, the same tone correction processing is applied to all the pixels by using a 1×1 dither matrix or a multi-level dither processing that makes the density level difference between dots not more than 1 is applied to the same input value.
As a technique to make moire difficult to occur in the halftone area while reproducing a lower density in the multi-level dither processing, a technique has been proposed such that in the low density part, the output values are distributed to the pixels in a predetermined order within the dither matrix so as to locally increase and in the high density part, the output values are distributed to the pixels one by one in a predetermined order within the dither matrix so as to uniformly increase (for example, Japanese Patent Application Laid-Open No. 2001-257893).